Differential-excitation studies of quasi-one-electron systems. II. Mg II 3²P alignment and orientation in Mg⁺-He, Ne, Ar collisions

For pt.I see ibid., vol.12, no.15, p.2529 (1979). The impact-parameter dependence of the Mg II 3 2 S-3 2 P emission probability and polarisation has been measured for 3-15 keV Mg + -He, Ne and Ar collisions by detection of the photons emitted perpendicular to the scattering plane in coincidence with ions scattered over a selected angle theta . From the four measured Stokes parameters, the emission probability and the degree of polarisation P have been determined. The two parameters lambda and chi describing the relative size and phase of the quantal excitation amplitudes for the magnetic sublevels were evaluated in the cases where the excitation is coherent. The degree of polarisation is high in all cases except for the Mg + -Ne system for distances of closest approach below 1.3 au, where molecular curve crossings due to promotion of neon 2p electrons cause a breakdown of the quasi-one-electron description. The parameters lambda and chi exhibit a strong dependence on the impact parameter, beam energy and target gas.